Neodymium magnets are a molding sintered product including neodymium (Nd), iron oxide (Fe), and boron (B) as main components, and exhibit excellent magnetic characteristics. The demands for these high-performance neodymium (Nd)-based bulk magnets are sharply increasing, but the imbalance problems between demand and supply of rare earth element resources have become a big obstacle to the supply of high-performance motors required for the next-generation industry.
Ferrite magnets have stable magnetic characteristics and are an inexpensive magnet used when a magnet having strong magnetic force is not required, and usually display black. Ferrite magnets have been used for various uses such as D.C motors, compasses, telephones, tachometers, speakers, speedometers, TV sets, reed switches, and clock movements, and are advantageous in lightweight and low prices, but have a problem in that the ferrite magnets fail to exhibit excellent magnetic characteristics capable of replacing expensive neodymium (Nd)-based bulk magnets. Therefore, there is a need for developing a novel high-performance magnetic material capable of replacing rare earth magnets.
MnBi is a rare earth-free material permanent magnet, and has a characteristic of having a larger coercive force than an Nd2Fe14B permanent magnet at a temperature of 150° C. because the coercive force has a positive temperature coefficient at a temperature interval of −123 to 277° C. Therefore, MnBi is a material suitable for being applied to motors which are driven at high temperature (100 to 200° C.). When compared to other magnets in terms of the (BH)max value which exhibits a magnetic performance index, MnBi is better than ferrite permanent magnets in the related art in terms of performance and may implement a performance which is equal to or more than that of rare earth Nd2Fe14B bond magnets, and thus is a material capable of replacing these magnets.
Meanwhile, sintering is a heat treatment intended to obtain mechanical and physical properties required for powder molded bodies by heating compressed or uncompressed powder molded bodies at a temperature which is equal to or less than the melting point of a main constituent metal element to allow bonds to be formed by the action of sufficient primary binding force between atoms among powders in the molded bodies which are initially maintained by only a weak binding force. That is, sintering refers to a process in which powder particles are subjected to thermal activation process to become a lump.
The driving force of sintering is to thermodynamically reduce the surface energy of the entire system. Since there is an excess energy at the interface unlike the bulk, the surface energy during the sintering is reduced in a process in which particles are densified and coarsened. The sintering process parameters are temperature, time, atmosphere, sintering pressure, and the like. The process in which particles are sintered generally goes through an initial bonding step in which particles are aggregated with each other to form a neck, a densification step in which blocking of pore channels and spheroidization, shrinkage, and termination of pores proceed, a subsequent coarsening step of pores, and the like.
Methods of sintering a molded body may be largely classified into atmospheric (normal pressure) sintering methods; or pressure sintering methods. Hot-press sintering, hot isostatic pressure sintering, and the like belong to pressure sintering methods. Among these sintering methods, pressure sintering has advantages in that the densification close to nearly 100% may be obtained by minimizing the amount of residual pores in a sample, the mechanical processability is excellent due to the pressurization during the sintering in the initial stage, and densified complex materials may be prepared, whereas the production costs are accordingly increased and the pressure sintering cannot be applied to continuous processes, so that it is difficult for the pressure sintering to be commercialized.
Throughout the present specification, a plurality of documents are referenced, and citations thereof are indicated. The disclosure of each of the cited documents is incorporated herein by reference in its entirety to describe the level of the technical field to which the present invention pertains and the content of the present invention more apparently.